This present invention relates to a combustion and control system for increasing the productivity and energy efficiency of regenerative furnaces, such as those used in high temperature heating and melting applications.
The typical system used for the melting of glass in industry is the regenerative furnace (or "glass tank"), which is constructed largely of brick and other refractories. In this glass furnace, glass is melted in a large refractory lined tank which is maintained at temperatures above 2750.degree. F. As the molten glass is withdrawn from the furnace, recycled glass and/or new raw material, depending on the desired quality of the product being produced, is added to make up the charge.
The glass bath is heated by a series of burners which can be fueled with natural gas, petroleum gas, fuel oil, or low BTU gas (such as coke oven gas). Each side of the furnace is equipped with a series of burner ports, each of which contains at least one burner which injects a stream of fuel into preheated air (1300.degree.-2000.degree. F.) introduced into the furnace through the port. This air is preheated in regenerators, which are usually constructed in brick.
The heat from the escaping flue gases is captured by regenerators and then recaptured by preheating combustion air, which is blown through the heated bricks of the regenerator and into the furnace. Every fifteen to twenty minutes these flows of exhaust gases and combustion air are alternated, thus drawing the combustion air up through the regenerator, which is now hot, and the exhaust gases up through the regenerator, which is cold. As the flows are alternated, the flame traverses the glass tank in opposite directions. This operation results in the recovery of heat from the exhaust gases which increases flame temperature beyond levels that can be achieved with ambient combustion air, increases furnace productivity (pull rate) and improves furnace thermal efficiency.
The fuel stream is mixed with the preheated combustion air to generate a high temperature flame. The hot products of combustion pass through the furnace, transferring heat to the load as well as to the furnace roof, which then radiates heat to the load. The exhaust gases are channelled through the opposite regenerator providing heat to the refractory brick. The flue gases then pass through a reversing valve to the furnace stack. The furnace production rate is typically limited by heat flux, which can be transferred from the flame to the load without overheating the furnace crown. An increase in flame luminosity is always desired to raise the radiative heat transfer from the flame to gain furnace throughput and thermal efficiency.
There are, however, problems encountered in using the standard-type regenerative furnace. Glass furnace "campaigns" (the time between major overhauls) can run for many months or even years. At the end of a campaign, much of the refractory in the furnace has deteriorated significantly and the regenerators in particular will need substantial rework. During the campaign the gradual deterioration of the refractory in the regenerator results in plugging of the regenerator, reducing the cross sectional area of the refractory brick exposed to the flow of exhaust gases and combustion air. The result is a reduction in heat recovery and therefore a decrease in the temperature of preheated combustion air delivered to the ports, which in turn decreases total heat input and furnace productivity.
Throughout the glass furnace campaign, various impurities and foreign matter will be carried out of the tank by the exhaust gases and deposited on the regenerators. This increased resistance to air and exhaust gases flows results in the deterioration of combustion air flow, so that the furnace will not be capable of maintaining the necessary maximum firing rate required for maximum production rates.
A number of problems arise as a result of the switching cycle used in regenerative furnaces. For example, a common problem with traditional regenerative furnaces is the undesirable cooling effect on the furnace interior of the incoming air stream used to purge combustible gases from the regenerator while switching from one regenerator to the other. During this switching cycle, which consumes at least 3-5% of an entire working campaign, the fuel stream is shutdown and combustion air at lower than furnace temperatures is delivered to the furnace from the process of purging the regenerators. This purging of flue gases from the regenerator is necessary during the switching cycle to establish proper air flow throughout the regenerator prior to restarting the fuel flow. The shut down of the burner and the purging of the regenerator negatively impacts furnace productivity. During the switching cycle, the purge air is taking heat from the load and furnace linings reducing production capacity and furnace efficiency.
Also, the switched bed nature of the regenerative air heaters results in less than optimum flame temperatures and reduced recaptured heat inputs during the latter portion of each firing cycle because of gradual cooling of the regenerators. At the beginning of a cycle the temperature of the combustion air supplied to the glass tank burner will be 1900.degree. F.-2400.degree. F. However, at the end of a cycle, this temperature may be down to 1600.degree. F.-2100.degree. F., which will result in lower flame temperatures and which will limit the amount of glass which can be melted.
There exists a need, therefore, for means for improving the heat transfer efficiency between the flame produced in the combustion air and the product to be heated or melted through improved flame luminosity.
There also exists a need for means for stabilizing the heat input at a maximum allowable level based upon the properties of the furnace refractory.
There exists another need for such means for improving heat transfer efficiency and for stabilizing the heat input at a maximum allowable level to overcome deterioration in heat input due to the regenerator plugging throughout the furnace campaign.
There also exists a further need for means for providing heat input by using an auxiliary fuel and oxidizing gas stream to prevent furnace cooling when the main fuel is shut down during the switching period.